1. Field of the Invention
The present invention relates to a method for forming oriented thermoplastic articles, and more particularly to a method and apparatus utilizing moveable blow pins for transporting blown pre-forms through a thermal conditioning region prior to the final blowing operation.
2. The Prior Art
In the art of blow molding thermoplastic articles, the so-called "blow and blow" technique has become widely used for molecular orientation. In such operations, a blowable plastic parison is positioned within a first mold cavity and blown to a pre-form configuration. Next, the pre-form is positioned within a second mold cavity, where it is blown to a configuration of the final container. This general overall process has undergone several refinements, including a sequential blow, stretch, and blow technique as disclosed in U.S. Pat. No. 3,781,395, which is incorporated by reference.
Even though the overall operation of U.S. Pat. No. 3,781,395 advanced the art of molecular orientation, this patent does not disclose a thermal conditioning means between the pre-form mold and final blow mold. Thus, the blown pre-form must be maintained in the pre-form mold for a sufficient period of time to bring the plastic material temperature to within the range for molecular orientation. Depending upon the material being used, an unnecessarily long dwell time may be required for the pre-form in the first mold cavity, thereby prolonging the overall "blow and blow" operation cycle.
U.S. Pat. No. 3,873,660, which is also incorporated by reference, proposes that the temperature of a blown pre-form be adjusted within a thermal conditioning chamber between the pre-form molding station and the final molding station. The apparatus disclosed in this patent, however, requires pre-form handling and trimming mechanism which can adversely affect the temperature for optimum molecular orientation.
Applicants here propose to overcome these disadvantages in the prior art by the use of a plurality of moveable blow pins from which the blow preforms suspend while in a thermal conditioning chamber in transit to the final blow molding station. Although moveable blow pins, per se, are disclosed in U.S. Pat. No. 3,599,280, and pre-form conveying systems are disclosed in U.S. Pat. No. 3,324,507, these patents do not provide the overall advantages, method, or apparatus of the present invention.